Selenium is a possible cancer preventive agent. A study is in progress to provide information on the pharmacokinetics of selenium in its prototype forms: sodium selenite and selenomethionine. This information is necessary for the determination of time and manner of administration. Integrated kinetic models are being used to interpret the study data. Various body pools have been hypothesized and rates of exchange between them estimated, as well as residence times. The models indicate important kinetic differences between selenite and selenomethionine. Alternative models were used to investigate one of the most important differences, the recirculation of the organic, but not the inorganic. The models have been modified and combined into a single model to better simulate dietary intake of selenium. The selenite model has been modified to take into account body stores of tracer, present at the time the dose was given in the pharmacokinetics study, as well as dietary tracer. The introduction of tracer extraneous to the dose resulted in substantial changes to many model parameters. This modification was accomplished via an iterative procedure in which model parameters were modified and body stores reestimated until the model fit was considered adequate and estimates of body stores converged.